The adaptation of insect vectors of human diseases to breed in human habitats
(domestication) is one of the most important phenomena in medical entomology. Considerable
data are available on the vector mosquito Aedes aegypti in this regard
and here we integrate the available information including genetics, behaviour, morphology,
ecology and biogeography of the mosquito, with human history. We emphasise the tremendous
amount of variation possessed by Ae. aegypti for virtually all traits
considered. Typological thinking needs to be abandoned to reach a realistic and
comprehensive understanding of this important vector of yellow fever, dengue and
Chikungunya.

“When to this is added the fact that the more closely a mosquito is associated with man, the
more is it the subject of prejudice and misconception, it follows that the prevailing
conception of Ae. aegypti in the minds of the general run of entomologists
may well be more remote from reality than in the case of most other mosquitoes.” Mattingly (1957)

As humans have grown in numbers and occupancy of the Earth, their habitats have encroached on
the native habitats of many species. One outcome is extinction of the invaded species, another
is evolution of “domestication” or commensalism, the breeding in human-occupied territory.
When this occurs for insects that require a vertebrate source of blood, the results can be
disastrous. These blood-requiring insects most often evolve a preference for the most
available and stable blood source: humans. Many major insect vectors of human diseases have
undergone this domestication process and now breed in close proximity with humans and take
human blood meals.

One consequence of this switch in taking blood meals from non-human animals, zoophagy, to
anthropophagy is that humans are challenged with infectious diseases once confined to animals.
Humans are a relatively recent member of the Earth’s biota having arisen less than 10 million
years ago. Blood-feeding insects have been around for hundreds of millions of years. Thus it
is safe to assume that the various infectious agents transmitted by insects have a long
history with non-human animals and that their infection of humans is a recently derived
phenomenon. Many human vector-borne pathogens today also infect animals; those that do not,
have close relatives infecting animals.

A second consequence of commensalism is the potential for the spread of vectors outside their
previous “native” range, i.e., becoming an invasive species. Because of all species, humans
occupy the widest range of habitats on Earth, once a species evolves the ability to co-exist
with humans they will likely be spread by the great mobility of humans. Lounibos (2002) provides an excellent synopsis of the importance of
invasiveness in insect vectors.

So, from a public health perspective, the evolution of vector domestication is an extremely
important phenomenon, yet has not received the close study it would seem to warrant. Here we
focus on Aedes aegypti , a widespread species of mosquito that has both
domestic populations as well as the ancestral type that still extant in sub-Saharan Africa.
This will be done in the context of ongoing work on the evolutionary genetics of this species.
Behavioural changes associated with domestication are particularly important and are
emphasised here. Because insect behaviour genetics was the focus of Alexandre Peixoto’s
brilliant, but all too short, career, this theme is a fitting tribute to his memory.

History of Ae. aegypti - While the official common name for this species is
the “yellow fever mosquito”, today it is of most public health concern as the major vector of
dengue fever. Due to an effective vaccine, yellow fever is of less concern worldwide, although
cases still occur ( Barrett & Higgs 2007 ).
Generally, Ae. aegypti is important in spreading viral diseases such as
yellow fever, dengue fever and Chikungunya.

Tabachnick (1991) reviewed many of the ideas about the
history of Ae. aegypti’s distribution throughout the world given the
information at that time. It is almost certain that the ancestor of the domestic form of
Ae. aegypti lived in sub-Saharan Africa. The larval habitat was likely
tree holes and non-human animals provided blood meals. Today, this ancestral form still exists
in forests and vegetated ecotones in sub-Saharan Africa ( Lounibos 1981 ) and is called by a subspecies name, formosus. In
addition to laying eggs in tree holes and preferring non-human blood, morphologically this
form is much darker than the form adapted to human habitats, although this morphology based on
scaling patterns is quite variable ( McClelland 1974 )
and, as will be clear later, is decoupled from the behavioural traits associated with urban
vs. sylvan breeding in different parts of the world.

Two scenarios have been put forward for the origin of the light-coloured domestic subspecies,
Ae. aegypti aegypti (for ease of communication, from here on we refer to
forest-breeding populations in sub-Saharan Africa as the classically defined
formosus subspecies as Aaf and the light coloured populations outside of
outside Africa as Aaa. However, as will become clear, this simple dichotomy masks the true
complexity of the species). Almost certainly Ae. aegypti came to the New
World on ships where conditions were such as to select for a domestic type. The two scenarios
differ in whether the species had already become domesticated prior to spread (i.e.,
pre-adapted to human transport) or became domesticated in response to transport. The species
was likely once more widespread including in forested northern Africa before the formation of
the Sahara Desert. As the north part of the continent dried over the last 4,000-6,000 years
forming the Sahara ( Kropelin et al. 2008 ),
populations along the northern coast and around the Mediterranean would have become isolated
from the sylvan form south of the Sahara. As the drying continued, the only reliable water
sources for northern populations were those found in human settlements. Interestingly, a third
subspecies, Aedes aegypti queenslandensis , was described as a particularly
light coloured form found in the Mediterranean Basin ( Mattingly 1967 ). As Ae. aegypti has been eradicated in the
Mediterranean Basin, it is not clear if queenslandensis still exists although
we do know it was certainly a domestic form.

Whether the domestication event preceded or occurred simultaneous with its introduction into
the New World, Ae. aegypti arrived soon after Europeans first arrived. Yellow
fever was known in sub-Saharan Africa much before 1400, but was not known in the New World
prior to European arrival. The first confirmed outbreak of yellow fever in the New World
occurred in the Yucatan, in 1648 ( McNeill 1976 ),
although yellow fever may have been in Haiti as early as 1495 ( Cloudsley-Thompson 1976 ).

The early trade between the Old and New Worlds has been described as “triangular” ( Murphy 1972 ). Ships from Portugal and Spain sailed to West
Africa to acquire slaves, brought them to the New World where they were exchanged for goods
that were brought back to Portugal and Spain. Whether the ships acquired Ae. aegypti
in West Africa or already had the domestic form when they originated in Europe is
not clear. Aaa as it occurs in the New World is not known in West Africa today, except perhaps
as a reintroduction into ports ( Brown et al. 2011
).

Evidence from DNA sequencing and large-scale single nucleotide polymorphisms (SNP) analyses
indicate that following introduction into the New World the species likely spread westward
across the Pacific into Asia and Australia ( Figure ).
Populations in the New World are derived directly from African populations, while
Asia/Australian populations are derived from New World populations. A second piece of genetic
information favouring Africa to New World to Asia/Australia is the level of genetic variation.
Table summarises the information. As would be
expected by two successive founding events, the amount of genetic variation decreases from
Africa to the New World and then again from the New World to Asia/Australia. The westward
expansion from the New World to Asia is surprising given that an eastern migration from East
Africa to Asia might be expected based on geography and the historic intensive trade between
India and East Africa. We have not yet seen genetic evidence of this, although it must be
noted that our sampling in Asia is sparse especially with regard to the Indian Subcontinent.
However, recent analyses of samples from Saudi Arabia are placed with other Asian populations
(A Gloria-Soria & JR Powell, unpublished observations). Data from allozymes indicated that
samples from India were not genetically different from those from Indonesia and Taiwan ( Wallis et al. 1983 ). So as far as we know the colonisation
out of Africa was unidirectional, westward (although see below). The timing of Ae.
aegypti colonisation of Asia is likely the late XIX century when dengue fever was
first reported and, importantly, in urban settings due to the arrival of the only urban dengue
vector, Ae. aegypti ( Smith 1956 ).
[The historic absence of yellow fever in Asia, despite the presence of Ae.
aegypti , remains one of medical entomology’s great mysteries. Several explanations
for this have been proposed ( Tabachnick 2013 )].

Evolutionary history of Aedes aegypti from single nucleotide
polymorphisms (SNPs) and sequenced nuclear genes. Bootstrapped neighbour-joining network
based on population pairwise chord-distances from 1,504 SNPs (left). Bayesian population
tree based on phased DNA sequences of genes listed in Table. Node support over 75% is
shown on relevant branches. East African populations are shaded in red, West and Central
African populations in pink, the Rabai domestic (called Aaa here) population in purple,
New World populations in dark blue and Asia-Pacific populations in light blue. Rooting
was inferred from DNA sequences of three nuclear genes from Aedes mascarensis
( Brown et al. 2013 ).

Why might East Africa not be a source for nearby Asia regions? Except for the unusual
situation in east Africa highlighted by Rabai, Kenya (discussed next), a domestic form of
Ae. aegypti capable of passive transport by humans may not have existed in
East Africa. Yellow fever was unknown or very rare in East Africa until recently ( Mutebi & Barrett 2002 ) and what epidemics occurred
have been sylvan and transmitted by Aedes species other than Ae.
aegypti ( Saunders et al. 1998 ). In fact,
Mutebi and Barrrett (2002) state that “…in West Africa, Ae. aegypti is
responsible for urban YF outbreaks, whereas in East Africa, Ae. aegypti has
never been incriminated in the transmission of YF virus”. Thus epidemiologic data indicate
East African aegypti are very different from West African and either are not
sufficiently associated with humans and anthropophilic and/or not as competent to transmit
yellow fever. The fact that East African aegypti are not favourable vectors
of human diseases indicates limited adaptation to human environments, perhaps precluding them
from surviving aboard ships for long periods as would be required for East Africa to Asia
migration on ships.

While this origin of present day Asian Ae. aegypti by colonization from the
New World is consistent with the current genetic data, an alternative scenario based on
historical considerations was proposed by Tabachnick
(1991) . The origin of domestic Ae. aegypti is posited to have
occurred in North Africa as described; whether this initial form was more similar to the
description of Aaa or subspecies queenslandensis is not known. The
introduction of domestic aegypti to West Africa may have occurred via human
trade at a time when introgression with sylvan aegypti occurred there,
resulting in the domestic populations and morphology now observed in West Africa. The
introduction of domestic aegypti into East Africa may have occurred much
later, including into the Rabai region, where domesticity allowed it to remain sympatric with
sylvan forms in this particular environment. Therefore domestic aegypti in
Asia, now corresponding to Aaa elsewhere outside of Africa, would be a later arrival since it
was a recent arrival to East Africa. This is also consistent with Smith’s (1956) observation
that Ae. aegypti likely arrived in Asia in the latter half of the XIX
century since urban dengue epidemics were unknown until then, until the arrival of the only
urban vector, Ae. aegypti . Figure powerful evidence arguing against the
likelihood of East Africa Rabai region as the origin for Asian Ae. aegypti .
A more direct route might have occurred to Asia from the North African ancestral Ae.
aegypti with the opening of the Suez Canal and the accompanying increase in ship
trade to the Indian subcontinent ( Tabachnick 1991 ).
This is consistent with the introduction of Ae. aegypti to Asia in the latter
part of the XIX century and consistent with what we know about human trade and migration that
might support Ae. aegypti migration. Further studies will be needed to
resolve these issues.

Perhaps most remarkably, the domestic form of Ae. aegypti that now exists
outside Africa throughout the tropical and sub-tropical world is a monophyletic group (Figure)
( Brown et al. 2013 ). The implication is that the
ancestral domestication event leading to the initial domestication of Ae.
aegypti occurred once and all populations outside Africa are descended from this
single lineage.

Sympatry of domestic and sylvan Ae. aegypti - A possible exception to the
general statements made above occurs along the east coast of Africa ( Teesdale 1955 , van Someren et al. 1958, McClelland 1973 ) best studied in the Rabai District of Kenya. Here both a domestic,
light coloured form more or less (see later) corresponding to Aaa breeds in stored water in
villages. Just a few hundred meters away, a form fitting the classical description of Aaf
exist in the vegetated ecotones. These two forms have remained genetically distinct from one
another over a period of at least 30 years ( Tabachnick et al.
1979 , Brown et al. 2011 ) and likely longer (
Mattingly 1957 ). There are no reproductive barriers
between the forms with hybrids and backcrosses perfectly fertile and, at least in the
laboratory, they randomly mate with one another ( Moore
1979 ). That these forms are truly sympatric is confirmed by finding the forest form
in the huts at certain times of the year ( Trpis &
Hausermann 1978 , Lounibos 2003 ). In addition
to morphology and larval site differences, these two forms from Rabai display distinct
differences in choice of host for blood meals, the indoor type preferring humans and the
sylvan form non-human mammals ( McClelland & Weitz
1963 , L McBride, unpublished observations).

Another trait of interest observed between the two forms from Rabai is oviposition
preferences. Ae. aegypti females lay eggs just above the water line of
natural pools (e.g., tree holes) or water in human-generated containers (e.g., flower pots,
bird baths, discarded tires). The eggs remain dormant until flooded with water. Presumably
this oviposition behaviour was adapted to natural conditions where water (rain) is
unpredictable. If a pool is drying, eggs remain dormant; if rain is plentiful water rises to
flood the eggs, they hatch and are more likely to have water long enough to undergo
development. Lorimer et al. (1976) showed that the Rabai indoor form preferred clay surfaces
such as the stored water jars in Rabai huts which is not the case with Aaa outside Africa.
Evidently the oviposition cues are tactile in this case as opposed to the usually assumed
olfactory ( Lorimer et al. 1976 , Lounibos 2003 ). Another unusual trait of East African indoor populations
of Ae. aegypti is that it is that larval development is dependent upon
permanent stored water. Indeed McClelland (1973)
considered that the temporal stability of larval sites is more significant than natural (e.g.
tree holes) vs. manmade containers because both are intermittently flooded by rain.

As can be seen in Figure and further documented in Brown et al. (2011, 2013), the domestic
form found in Rabai, while morphologically and behaviourally (host choice) Aaa, is genetically
distinct from other Aaa. What is the origin of this unique indoor form Ae.
aegypti in Rabai? The phylogeny in Figure suggests this is an old lineage not
closely related to other Aaa in the New World. Is this a surviving remnant of the Ae.
aegypti queenslandensis described by Mattingly
(1957) and once widespread around the Mediterranean? Indoor-breeding Ae.
aegypti in coastal Kenya had been described as queenslandensis by
Mattingly (1957) . This subspecies was dependent upon
stored water in human structures and its demise in the Mediterranean Basin coincided with the
introduction of indoor plumbing in the early XX century ( Curtin 1967 , Holstein 1967 ). As noted above
the dependence on permanently stored indoor water persists in present day East African indoor
populations ( McClelland 1973 ).

Oviposition and “reversion” - As emphasised, the spread of Ae.
aegypti out of Africa required the species to adapt to human environments with
larval development in human-generated containers. Obviously this required a change in
oviposition behaviour of ancestral sylvan females to first, enter human disturbed, even urban,
environments and, second, to oviposit on metal, clay, rubber, etc., all of which would have
been absent in its ancestral habitat. The adaptation for oviposition preference may have been
part of the overall evolution of domesticity that likely occurred in North Africa when
ancestral sylvan Aaf became isolated from sub-Saharan Africa due to the Sahara Desert ( Tabachnick 1991 ). In general oviposition choice in
mosquitoes is largely due to volatiles produced by the microorganisms in the larval water
(although see exception mentioned earlier). Thus, as long as appropriate volatiles are
produced by a standing pool of water, an opportunistic species like Ae. aegypti
may oviposit there.

This is supported by situations where this domestic form outside Africa has reverted to
developing in natural water. This has occurred mostly on islands or other isolated sites.
Chadee et al. (1998) report 12 types of natural habitats where Aaa can be found in Jamaica,
Puerto Rico and Trinidad including rock holes, tree holes, leaf axils, bamboo joints and
coconut shells. Larvae developing in rock holes has been documented on the east coast of
Africa ( Trpis 1972 ) and in Anguilla ( Wallis & Tabachnick 1990 ). Aaa has been observed
ovipositing in tree holes in New Orleans [cited in Wallis and Tabachnick (1990)].

In the case of rock hole larval sites on Anguilla, allozyme gene frequency differences were
found in Anguilla between populations breeding in human-generated containers and rock hole Aaa
a few kilometres apart ( Wallis & Tabachnick 1990 )
and the mosquitoes in the two habitats were also significantly differentiated with regard to
development time and insecticide resistance ( Tabachnick
1993 ). No difference was found in oviposition preferences.

[While not a natural site, as further evidence of the flexibility of Aaa on islands, larvae
are found in septic tanks in Puerto Rico ( Barrera et al.
2008 ), a niche more common for Culex pipiens and other mosquito
species. No genetic differences were observed between surface and septic tank Aaa in Puerto
Rico ( Somers et al. 2011 )]

This demonstrates that the species has remained adaptively flexible, maintains significant
genetic variation for different life history traits and that breeding in human-generated
containers is not a fixed trait outside of Africa. Aaa remains opportunistic, capable of
rapidly responding to changes in environments. In the case of larval breeding sites,
relatively few species of mosquitoes occur on islands, so not all potential mosquito larval
niches are filled. In such cases, the invasive Aaa initially introduced into domestic
habitats, spills over to occupy the empty natural niches. This is not in any sense a true
“reversion” to the ancestral sylvan type; rather these are simply feral populations of what
are genetically Aaa.

Genetics of morphology - While morphology, in particular the colour of scales
on abdominal tergites and background cuticle coloration, were important in differentiating the
classical Aaa and Aaf, the detailed work of McClelland
(1974) initially called such a simple dichotomy into question. He demonstrated that
scaling patterns are highly variable within populations (as well as between). Many of the
patterns have close resemblance to single gene Mendelian mutations known for this species
(Munstermann 1993). In this regard the observations of Verna and Munstermann (2011) are
instructive. Morphologically exceptional specimens were collected from a bucket on the island
of Antigua that included a remarkably gold form. “The Antigua variants demonstrated morphology
comparable to previously described mutations...” ( Verna &
Munstermann 2011 ).

Thus the evidence is that scale pattern is a genetically highly variable character within and
between populations of Ae. aegypti occupying various ecological niches; many
of these patterns are due to variation at simple single Mendelian genes and not some complex
of interacting genes that would take longer to evolve. Genetic relatedness as indicated by
multiple molecular markers such as allozymes ( Wallis et al.
1983 ), microsatellites ( Brown et al. 2011 )
and 1,504 SNPs ( Brown et al. 2013 ) often do not
coincide with morphological similarities.

The conclusion is that variation in colouring exists within and between Ae. aegypti
populations and only in some instances, in specific regions of the world, is that
variation also indicative of the behavioural traits that lead to differences in adaptations.
To our knowledge, no one has suggested an adaptive explanation for scaling colour variation in
Ae. aegypti .

West Africa - The most dynamic situation with regard to domestication of
Ae. aegypti is occurring in West Africa. Ae. aegypti have
begun to breed in domestic habitats and it is clear that this is an independent evolutionary
domestication from that leading to the spread of Aaa out of Africa. Domestic-breeding
populations in West Africa have their closest relatives in sylvan populations in the same
vicinity ( Paupy et al. 2008 , 2010, Brown et al. 2011 ) and are not closely related to Aaa
outside Africa. While some workers describe domestic populations in West Africa as Aaa based
on the presence of pale scales on the first abdominal tergite ( Huber et al. 2008 ), its overall morphology especially cuticle colour is
much darker than Aaa outside Africa and is more similar to sub-Saharan Aaf. The situation in
Senegal is more complicated as there is evidence that Aaa from outside Africa has migrated
back to Senegal ( Brown et al. 2011 ) and thus domestic
forms in Senegal exhibit some characteristics similar to Aaa outside Africa ( Sylla et al. 2009 ).

The independent domestication occurring in West Africa opens the exciting possibility of
studying the dynamics and genetics of this important event. This is almost certainly quite
recent coinciding with expansion of human habitats and cities in West Africa and there are
multiple independent incidents of sylvan populations moving into cities.

Epidemiology - In addition to all the traits so far discussed which are of
importance from an evolutionary and ecological perspective, there is also considerable genetic
variation in traits of importance for public health within and among populations of
Ae. aegypti , in particular for their ability to transmit arboviruses,
yellow fever and dengue in particular [reviewed in Black IV et al. (2002)]. Generally,
populations described as Aaf have lower competence to transmit both dengue and yellow fever
viruses than populations of Aaa. This brings up the intriguing possibility that the
domestication process of mosquitoes has been accompanied by increase competence to transmit
human viruses. Tabachnick (2013) posited out that
vector competence is likely the result of the effects of adaptations for other functions not
having anything to do with vector competence. In this view the adaptations accompanying
domestication, whatever they are, have side effects that result in greater competence of Aaa,
for example, to transmit yellow fever and dengue viruses.

Another reason for the correlation between the competence of domestic mosquitoes for human
virus transmission could be adaptation of the virus to the mosquito. Once a mosquito like
Ae. aegypti evolves to use humans as blood meals, there would be pressure
on human arboviruses to adapt to this species of mosquito for transmission, in particular to
the particular mosquito genotypes feeding on humans. It is clear with the related mosquito
Aedes albopictus, that the arbovirus Chikungunya rapidly evolved to a new
mosquito host ( Tsetsarkin et al. 2011 ). So when an
ancestrally zoophilic mosquito evolves anthropophily and introduces new viruses, the virus
evolves to be efficiently transmitted through the human hosts and those mosquitoes feeding
preferentially on humans. Others ( Moncayo et al. 2004
, Vasilakis et al. 2011 ) have also emphasised the
importance of viral genotype in emergence of dengue. Evidently, the longer the evolutionary
history of association of a mosquito with a virus, the more efficient the virus replicates in
the arthropod host ( Moncayo et al. 2004 ).

Variation abounds - The above emphasises just how much variation exists
within the single species Ae. aegypti. This is not unique to this vector as
similar studies of insect vectors of disease have almost always revealed comparable variation
( Tabachnick 2013 ). In the case of Ae.
aegypti , one might make a partial list of such variable traits: (i) colour and
pattern of scaling, (ii) host choice for blood meal, (iii) oviposition choice, (iv) larval
sites, (v) egg dormancy, (vi) development time and (vii) competence to vector viruses.

These traits have both genetic and environmental components. The discussions argue that, at
least to a large degree, these traits vary independently and thus are not always concordant.
Considering this, it quickly becomes apparent that any attempt at categorising this species
into two or three “subspecies” or other taxonomic unit is folly. While the classical
definitions and designations of Ae. aegypti aegypti , Aedes aegypti
formosus andAe. aegypti queenslandensis were useful at one time and may sometimes still
be useful in efficient communication, developments in our understanding of the genetics and
behaviour of this species have revealed the extent that this is a gross oversimplification of
the true situation and represents typological thinking, discarded by most modern biologists.
While we base this conclusion on recent genetic findings, an insightful early Ae.
aegypti expert, McClelland (1967) wrote:
“...despite the population differences, Ae. aegypti cannot be split into
definite infraspecific entities. In conclusion, Ae. aegypti may best be
interpreted as a polymorphic rather than a polytypic species.”. In the 45 years since, this
advice has often been ignored, even in recent times.

Acknowledgements

Alexandre Afranio Peixoto was the academic grandchild of one of the authors (JRP, the PhD
mentor to Louis Bernard Klaczko). JRP had the privilege of introducing Alexandre Afranio
Peixoto as an academic “grandson” at an international meeting in Greece in 2011; Alexandre
Afranio Peixoto responded with a big smile. To Julia E Brown and many other colleagues too
numerous to list, for contributing to collection of more recent data from the Powell lab,
and to Leon Lounibos, for making numerous insightful and detailed suggestions that greatly
improved this paper.

This is an Open Access article distributed under the terms of the Creative
Commons Attribution Non-Commercial License, which permits unrestricted non-commercial
use, distribution, and reproduction in any medium, provided the original work is
properly cited.